The present invention relates to a preamplifier for use in a magnetic storage device having a magnetoresistive head. More particularly, the present invention relates to a preamplifier that is configured to provide a high input impedance for reducing lowpass filtering affects at the preamplifier input terminals thereby allowing the preamplifier to accommodate higher data rate signals from the magnetoresistive head.
Magnetic storage devices such as disc drives make use of a transducer for transferring information between the transducer and a magnetic media. One type of transducers that has been used in magnetic storage systems is the magnetoresistive (MR) or magnetostrictive transducer. The magnetoresistive transducer or head has the property that the resistance varies with magnetic flux passing through the head. Magnetic storage devices have utilized or exploited this relationship between the resistance of a MR head and the flux passing through the head to extract or read information from a magnetic storage media.
The MR head is often positioned proximate the magnetic media. As the magnetic media is moved past the MR head, the MR head senses changes in flux orientation produced by charged particles on the magnetic storage media as described in U.S. Pat. 4,191,977 to Lewkowicz, incorporated herein by reference. These changes in flux orientation are indicative of, among others, information stored on the magnetic media. Therefore, the resistance or changes in resistance of the MR element is indicative of information on the magnetic media.
The resistance of an MR element is, in part, a function of the strength of the magnetic flux to which it is exposed. Since the resistance of an MR element varies with magnetic flux, the current through, or the voltage across, the MR head is based not only on the data written on the magnetic media, but also the distance between the element and the magnetic media. In addition., temperature and process variations also affect the steady state resistance of the MR element. Accordingly, it is desirable to produce a preamplifier output signal that varies with changes in the magnetic flux, without varying significantly due to changes in the steady state resistance.
The MR heads are often biased with a constant current. As the magnetic flux passing through the MR head varies, a preamplifier detects the resistance changes of the MR head by detecting a change in voltage across the head. The output of the preamplifier is then provided to a second amplification stage and decoding circuitry for further processing.
A previously used preamplifier configuration for use with MR heads is a common base configuration as described in copending U.S. patent application Ser. No. 07/993,316 to Smith, inventor of the present invention, filed Dec. 18, 1992. The common base configuration has inherently low input impedance and is used to sense changes in current through the MR head. The common base preamplifier is coupled to the MR head using a coupling capacitor or other electronically coupling using a first and second feedback loop. The first feedback loop eliminates differential DC and low frequency components from the differential output of the common base preamplifier. The second feedback provides a common mode DC and low frequency component of the output signal to equal an externally supplied reference voltage.
One problem associated with the use of low input impedance preamplifier is that leads used to connect the MR head to the preamplifier input have an inductance that, together with the input impedance of the preamplifier, have a lowpass filtering effect at the preamplifier input. This low pass filtering affect tends to attenuate high frequency signals from the preamplifier input thereby reducing the bandwidth of the preamplifier. Disc drives having high data rates between the MR head and preamplifier input require signals having fast transition times. Signals having fast transition times have more high frequency components and require greater bandwidths. Therefore, it is necessary that any lowpass filtering effects be minimized or eliminated to improve the preamplifier bandwidth so that the data rate can be maximized.